The present invention relates to high speed transmission lines and, more specifically, to transmission line circuits employing flex circuits.
Conventional transmission line flex circuits typically employ one of two configurations, a microstrip construction shown in FIG. 1A and a stripline construction shown in FIG. 1B. In the microstrip (i.e., single-sided) construction, a signal trace 102 is separated on one side from a ground plane 104 by an intervening dielectric 106. In the stripline (i.e., double-sided) construction a signal trace 152 is separated on both sides from ground planes 154 and 155 by surrounding dielectric 156.
Significant limitations associated with such transmission line circuits relate to the nature of the dielectric material. That is, the commonly employed materials such as polyimide (Er=3.5) and liquid crystal polymer or LCP (Er=3.0) present a significant impediment to the propagation of wave fronts along the transmission lines due to their relatively high dielectric constants, Er. As a result, conventional flex circuits have low maximum bandwidth (e.g., −3 dB @ 3 GHz for 2-inch long traces), and unacceptably high DC resistance due to the narrow trace widths possible with conventional dielectric materials. In addition, if the thickness of these dielectric materials is to be kept within a range appropriate for most applications, only very narrow traces are possible if the transmission lines are to remain within the typical 50 ohm transmission line design rules. Due to their width and high resistance, these narrow traces result in transmission lines with low current carrying capacity.
Other materials having lower dielectric constants, e.g., Teflon and Teflon-like materials, have been proposed to replace higher dielectric materials. However, these constructions have been reported to be very fragile and expensive.
It is therefore desirable to provide transmission line circuits in which the transmission lines experience a much lower dielectric constant than with conventional constructions.